Methods for producing glycol ether esters via esterification of glycol ether alcohols are well known in the art.
For instance, lower boiling glycol ether esters such as those containing alkoxy groups of one or two carbon atoms, e.g. 1-methoxy- 2-propyl acetate and 2-ethoxyethyl acetate can be readily prepared by reacting the corresponding glycol ether alcohol with carboxylic acid in the presence of an acid catalyst. However, such types of acid catalyzed direct esterifications are generally complicated by numerous side reactions that produce significant amounts of undesirable by-products, thereby minimizing the conversion efficiency of the esterification process to its desired product ester. Lower reaction rates that might produce lower amounts of such by-products is obviously not a perfect solution to the problem, nor are complicated purification type separation techniques as disclosed e.g., in U.S. Pat. Nos. 4,544,453 and 5,202,463. Moreover, increasing the employable amount of acid catalyst (e.g. sulfuric acid) in order to increase the rate of reaction serves only to increase the amount of such by-product impurities. Such an increase in undesired by-products above the limit of tolerable acceptable amounts can significantly discolor the desired ester product and cause an early shut down of the esterification unit due to the concentration build-up of such by-products that may make their way back to the reactor via recycle. Thus, heretofore producers of such product esters have had to compromise and carry out the esterification process at whatever reaction rate coincides with the production of a tolerable acceptable amount of by-products when employing such conventional acid catalysts.
On the other hand, higher boiling glycol ether esters such as those containing alkoxy groups of three or four carbon atoms, e.g. 2-butoxyethyl acetate or diethylene glycol monobutyl ether acetate, are not readily commercially preparable via such a direct acid catalyzed esterification process. Such is due to the fact that such heretofore known acid esterification catalysts as sulfuric acid and p-toluene sulfonic acid are not stable for prolonged reaction periods at the high temperatures that are generally required to satisfactorily produce such higher boiling product esters. Such conventional acid catalysts break down and decompose after only a short period of time at such high reaction temperatures. Consequently, such higher boiling product esters are only commercially readily preparable via transesterification, as oppose to direct esterification described above. However, such transesterification processes are notoriously slow, complicated and expensive processes that require the use of an extra-value chemical to supply the ester group of the desired ester product, as disclosed e.g., in U.S. Pat. No. 3,700,726.
U.S. Pat. No. 5,231,222 is directed to the use of long chain alkyl benzene sulfonic acid catalysts to esterify alkyl alcohols with carboxylic acid to produce product esters. However, said patent is silent with regard to producing glycol ether esters which are water-soluble products derived from glycol ether alcohols and which are entirely different from the water-insoluble product esters obtained from alkyl alcohols.